[test] Rearrange Travis CI setups

- Tests on OS X.

- Tests with -m32.

- Tests with Valgrind. Note that, as of July 2016,
  - Open MPI is not Valgrind-clean,
  - MPICH is not in the whitelist of apt packages on Travis CI.
  So we need to run "valgrind-parvorm" test on OS X with brewed MPICH.

- Currently, many build jobs are disabled because they fail (e.g.,
  valgrind tests) or not so important. Enabled build jobs are:
  - linux-gcc-form
  - linux-gcc-tform
  - linux-gcc-parform
  - osx-gcc-form
  - osx-gcc-tform
  - osx-gcc-parform
  Note that gcc on OS X is essentially clang.